This is Dr David
with a history of thyroid cancer, endocrinologist give higher doses of thyroid hormone medications to suppress the TSH levels down low to decrease the chance of TSH levels causing growth of rogue cancer cells
but since you have been 45 years out from your thyroid cancer, you most likely don't have thyroid cancer worries anymore and your doctors now can safely reduce your thyroid hormone dose.
usually dropping thyroid hormone doses that much will not cause fever.
but can cause a bit of fatigue.
your fever and nausea could be from a viral infection.
you can use tylenol for your fevers.
make sure you drink plenty of water.
most likely your fevers and headaches and nausea symptoms will get better.
you will need to get your thyroid hormone levels checked again in 6-8 weeks.
you want your TSH levels between 0.3 and 3.0 range.
lower back pain usually is not due to drops in thyroid hormone doses either.
let me know if you have other questions.
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